imporve `cargo install` instructions

[lolbinarycat]

1. recommend cargo install --locked fend instead, the fact that respecting lockfiles isn't the default for cargo install is often considered a mistake, as it makes supply-chain attacks easier, since new installations of the package will automatically update to the newest compatible version
2. show command for how to build with rustls instead: cargo install --locked fend --no-default-features --features rustls. people may prefer rustls over openssl for several reasons, from security to ease of compilation.